The U.S. Congress completed its own review period of the deal a month ago with Democrats blocking a Republican effort to vote against it.

Iran and the group that includes the United States, Britain, China, France, Russia and Germany reached the agreement in July after long-running negotiations that had to be extended past multiple deadlines.

Head of Iran's Atomic Energy Organization Ali Akbar Salehi, bottom, speaks in an open session of parliament while discussing a bill on Iran's nuclear deal with world powers, in Tehran, Iran, Oct. 11, 2015.

Under the deal, Iran will limit its nuclear program in exchange for relief from sanctions that have badly hurt the country's economy in recent years.

The negotiations were conducted to address allegations that Iran was working to develop nuclear weapons.

Iran has long denied those charges, insisting its program is for peaceful purposes such as medical research and generating power.

Iranian conservative lawmaker Bijan Nobaveh Vatan holds up a paper with writing in Persian reading, "Opponent of the JCPOA," in an open session of parliament while discussing a bill on the deal, in Tehran, Iran, Oct. 11, 2015.

The big looming next step for the deal to go forward is a report from the International Atomic Energy Agency on its more than decade-old investigation into possible military dimensions of Iran's nuclear activity.

IAEA chief Yukiya Amano is due to deliver his report by December 15.

The IAEA is in charge of monitoring whether Iran lives up to its part of the international nuclear deal, and only after it says Iran is complying will any sanctions begin to be lifted.